Hi Hamish,
given that we currently have no way to test another port of v8, we are very hesitant to extend support to Haiku. We realize that this also applies to some other ports backed platform-*.cc files. A lot of the ports contain duplicate code and are not well-tested. This is not a good place for us to be, and we are rethinking the way to approach platform ports that are not maintainable by us. Please understand that we would hate to spread thin our limited resources in order to support a long tail of different platform ports. Therefore I'm sorry to decline going the direction of adding another platform port.
However, since V8 is open source, you are welcome to provide and maintain a patch set or even fork for Haiku.
Best Regards,
Yang